Transcriptional activity in yeast strongly correlates with promoter occupan
cy by general factors such as TATA binding protein (TBP), TFIIA, and TFIIB,
but not with occupancy by TBP-associated factors (TAFs). Thus, TBP exists
in at least two transcriptionally active forms in vivo. The TAF-containing
form corresponds to the TFIID complex, whereas the form lacking TAFs corres
ponds to TBP itself or to some other TBP complex. Heat shock treatment alte
red the relative utilization of these TBP forms, with TFIID being favored.
Promoter-specific variations in the association of these distinct forms of
TBP may explain why only some yeast genes require TFIID for transcriptional
activity in vivo.